Patent Document 1 discloses an interference observation apparatus which acquires an interference image of an observation object. This interference observation apparatus, using an optical system of a Michelson interferometer, splits incoherent light output from a first light source into first split light and second split light, reflects the first split light by the observation object, combines the first split light and the second split light, and acquires an image of interference light generated by the combining.
Further, the interference observation apparatus performs feedback control on an optical path difference between two optical paths in the Michelson interferometer on the basis of a detection result of the interference light using a second light source which outputs coherent laser light.
Here, a technique of keeping the optical path difference constant between two optical paths in the interferometer by the feedback control on the basis of the interference light detection result is called “phase lock”. A technique of changing a value of the optical path difference kept by the phase lock using the feedback control is called “phase shift”.